As conventional film production technology, there is known an electrochemical vapor deposition method which Isenberg et al. developed for the manufacture of stabilized zirconia film for SOFC (cf. Japanese Patent Kokai Publication 61-153280). In the production of an yttria-stabilized zirconia film on a porous support by this electrochemical vapor deposition method, one side of the porous substrate is exposed to water vapor (H.sub.2 O) including oxygen (O.sub.2) or hydrogen (H.sub.2) and the other side to a gaseous mixture of zirconium tetrachloride (ZrCl.sub.4) and yttrium trichloride (YCl.sub.3).
In this film production method, the oxygen source gas molecules diffuse through pores of the support and react with the metal halide gas on the other side of the support to produce a thin film of yttria-stabilized zirconia on the surface of the support (CVD process). As this film grows, the pores of the support are blocked and the oxygen source gas molecules then cannot pass through the support any longer. However, because the yttria-stabilized zirconia film thus formed is ionically conductive, the oxygen source gas molecules are able to drift through the film in the form of oxygen ions (O.sup.2-). Here, the O.sup.2- concentration gradient caused in the film by the difference in oxygen partial pressure between two sides of the film provides a driving force for ion transport. Therefore, the O.sup.2- ions diffuse through the film to meet with the metal halide gas to sustain the growth of the film (EVD process).
The electrochemical vapor deposition method, consisting of the above-mentioned two stages, viz. CVD and EVD, has the disadvantage that since the ion transport in the latter EVD process is dependent on the intramembrane concentration gradient of O.sup.2- ion, the film formation in the EVD process is slow. This is because negative charges left over by O.sup.2- ions accumulate on the growth side of the film and the potential barrier produced by these surface charges seriously interferes with the transport of other O.sup.2- ions. In order to improve the driving force of reaction, it might be contemplated to increase the difference in oxygen partial pressure between the two sides of the growing film but if the oxygen partial pressure on the metal halide gas side is reduced too much, the resulting film will have a surface composition deviating from the expected composition.
The present invention has been accomplished with the above-mentioned circumstances in view. Thus, the object of the invention is to provide a method of producing a film which provides a desired surface composition while improving the rate of film formation in the EVD stage of the conventional electrochemical vapor deposition method.